An unexpected surge in joblessness is piling pressure on the coalition's flagship £5bn back-to-work scheme, putting huge financial stress on the companies and charities contracted to deliver it.

Official figures published on Tuesday show that 370,000 long-term unemployed people were referred to the programme in the first five months of its existence, far more than official projections.

Firms delivering the programme developed business plans based on government estimates that 605,000 individuals would come on to the scheme over 2011-12, but at current rates of referral the total number for the period could be nearer to 850,000.

The figures, published by the Department for Work and Pensions (DWP), show that despite the overall volume of referrals increasing, the number of individuals currently on long-term sickness benefit coming on to the scheme has slowed to a trickle, putting further pressure on providers' budgets.

Working with this category of people – who have been on employment support allowance (ESA) for long periods and are regarded as the most difficult to get into work – is seen as crucial to the effectiveness of the programme.

The long-term jobless are potentially the most lucrative element of the work programme for providers. Firms are paid proportionately higher rates for getting this group into sustainable employment, while the Treasury stands to make significant savings on the benefits bill if the programme is successful.

The programme was designed by the coalition to get up to 1 million long-term unemployed people back into work in the first two years. Ministers appointed 18 predominantly private corporations as "prime providers" to run the scheme in a series of regional contracts starting in 2011.

"Indicative" early data put out by providers earlier this month suggested 20% of work programme participants were being helped into a job, substantially less than the government's 40% target.

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Kirsty McHugh, the chief executive of the Employment Related Services Association, which represents work programme prime providers and subcontractors, said that although it was early days, the combination of unexpectedly high volumes and the lack of ESA referrals was putting a "big stress" on her members.

Although no prime providers were reporting that they would not be able to cope, they were having to re-examine their business models, while charity subcontractors – many of whom are dependant on getting ESA clients referred to them – were being put in a vulnerable position, she said.

McHugh said the low numbers of ESA referrals meant that those people would not be able to "access the specialist support to find and sustain employment to which they are entitled".

The latest figures will encourage critics, who fear that the deteriorating job market will make the programme unsustainable in its present form. Last month a National Audit Office report criticised the work programme for being "over-optimistic" and said it was likely to help only 25% of those out of work, rather than the estimate of 40%.

They say large numbers of appeals against the WCA – many of them successful – have brought the appeals tribunal system close to collapse, and have called on the government to increase the flow of ESA clients to the programme.

Ministers trumpeted the scheme as a "massive boost for the big society" but Tuesday's figures will stoke fears that many charity subcontractors will go bust or be forced to leave the programme as a result of being starved of ESA referrrals.

Sir Stuart Etherington, chief executive of the National Council for Voluntary Organisations, said it was disappointing that the DWP data did not reveal the number of referrals to subcontractor charities.

"The number of referrals to the customer groups more traditionally associated with voluntary sector provision appear low, reinforcing fears of our special interest group that some customers are being deprived of the correct level of support needed to make them job-ready.

"It is disappointing that the referrals data does not drill down further than prime contractor level. We have heard worrying evidence of voluntary sector subcontractors receiving very low levels of referrals, so more thorough data could have really helped to pinpoint whether the sector's degree of involvement in the work programme tallies with previous assurances made by government.

"While the data being published today is not as comprehensive as we would like, there is enough information here to suggest that many of the concerns highlighted by our special interest group remain real issues."

Chris Grayling, the minister for employment, said: "I am encouraged that so many people have been referred to the work programme in the first five months to get the support they need to move into sustained work. This is a long-term programme and in the autumn we will release figures that show how many unemployed people have gone into sustainable employment."